In general, closed circuit televisions (CCTVs) installed in the indoors or the outdoors of departments, banks, exhibition centers, and factories as well as typical houses have been variously used in order to perform antitheft functions and determine the operating state of machine, the process flow, or the overall situation.
The CCTVs have been installed in a specific place to monitor all situations, which are happened in the place, at a remote plate. To this end, the CCTVs include image transmitters and display units to receive signals transmitted from the image transmitters and supply the signals to display apparatuses.
Meanwhile, generally, a digital camera is similar to a typical camera in that the digital camera employs the structure of the optical mechanism of the typical camera. However, the digital camera makes a difference from the typical camera in that the digital camera receives an image by using a charge coupled device (CCD) image sensor instead of films, and converts image signals into digital data to be stored in a memory in the form of a graphic file.
Through the digital camera, a user can instantly recognize the photographed image through a display screen image. In addition, the user can variously process the image photographed through the digital camera by editing or outputting the image by using a computer. If necessary, the user can print out the image without the complex processes such as film development and printing processes, so that the digital camera has the wide utilization.
FIG. 1 is a schematic view showing a camera according to the related art, and FIG. 2 is a view showing the image state according to the related art. FIG. 3 is a graph showing the state of light according to the related art, and FIG. 4 is a view showing an image photographed by using a camera according to the related art.
Referring to FIG. 1, the camera includes a first lens 1 having at least one surface, to which a diffractive optical element (DOE) is applied, and having a negative refractive power, a second lens 2 having at least one surface, to which the DOE is applied, and having a positive refractive power, an aperture 3 to adjust the quality of light between the first and second lenses 1 and 2, and an optical low pass filter (OLPF) 4 to pass a low frequency band between the second lens 2 and an imaging surface of an image sensor 5 and to cut off a high frequency band equal to or greater than a Nyquist frequency.
However, as shown in FIG. 2, if the lens having the DOE is used, extra light such as zero-order light or secondary light is focused on the imaging surface of the image sensor 5 in addition to primary light allowing an image to be exactly formed on the imaging surface of the image sensor 5. Accordingly, flare may be caused in the image.
In other words, as shown in FIG. 3, the secondary light and the zero-order light are produced about the primary light. As the intensity of the primary light is increased, the intensities of the zero-order light and the secondary light are increased.
In other words, if the lens is manufactured by using the DOE, an image shown in FIG. 4 is expressed. The image has the flare caused around a light source, so that the image quality of the image is degraded.
The flare is caused because the zero-order light and the secondary light are not exactly focused on the imaging surface. If the brightness is reduced in order to reduce the flare, the gray scale of a dark portion of the image is reduced, which serves as another cause to degrade the image quality.
Therefore, a technology of reducing the image flare to inhibit the image quality from being degraded is required.